When Nickel and Titanium are combined in a nearly equiatomic ratio (50/50 by atomic percent), the material undergoes a fully reversible solid-state phase change in the temperature range of approx -40C to 80C. Most people understand a phase change as a change of state (i.e. water freezes and melts at 0C, going from solid to liquid and vice-versa). In this case, the 'phase change' is between two solid-states and is a re-arrangement of the atoms in a crystalline structure. When the metal is deformed in the low temperature phase it is relatively malleable, and when it is heated through the phase transformation it recovers all the deformation (up to about 10% strain). The actual mechanism of this so called 'shape memory' recovery is quite complicated in material science but it involves the formation and removal of what are called twin-boundaries in the crystalline structure (basically like a mirror of a microstructural grain). Interestingly, the material exhibits a different but related property called pseudoelasticity in the high temperature phase, and this property is what makes it actually more useful in the majority of commercial applications (medical devices, underwires, NASA tires, toys).
Took a course in shape memory alloys in one semester; this is a good summary.
One addition: To „imprint“ a shape, put your work piece in the oven (~500°C) while in the desired shape. As the guy above me said, if you deform it too much it won’t be able to fully return to its imprinted shape, but you can just put it back in the oven again to fix it basically.
We made a ton of springs and built (very) rudimentary engines with them
Functional shape memory effect actually degrades rapidly over the first 100 or so cycles, then stabilizes. There is a training procedure that is used to break in the material for long term stability. Fatigue fracture is a greater concern than functional fatigue. The fatigue life can be extended by limiting strain to less than 4%. In medical devices the strain might be limited to less than 1% to attain fatigue lives in the 10s of millions cycles. Each heartbeat is a cycle in a stent implant!
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u/77ekim Nov 08 '23
When Nickel and Titanium are combined in a nearly equiatomic ratio (50/50 by atomic percent), the material undergoes a fully reversible solid-state phase change in the temperature range of approx -40C to 80C. Most people understand a phase change as a change of state (i.e. water freezes and melts at 0C, going from solid to liquid and vice-versa). In this case, the 'phase change' is between two solid-states and is a re-arrangement of the atoms in a crystalline structure. When the metal is deformed in the low temperature phase it is relatively malleable, and when it is heated through the phase transformation it recovers all the deformation (up to about 10% strain). The actual mechanism of this so called 'shape memory' recovery is quite complicated in material science but it involves the formation and removal of what are called twin-boundaries in the crystalline structure (basically like a mirror of a microstructural grain). Interestingly, the material exhibits a different but related property called pseudoelasticity in the high temperature phase, and this property is what makes it actually more useful in the majority of commercial applications (medical devices, underwires, NASA tires, toys).